Address timing issue in forward system test.

Job #754841 failed for 6593cf0b:

I've just had 5 runs fail on this.

I:forward:checking that priming queries are not forwarded (15)
I:forward:failed

The test should wait for the reply packet to avoid checking while the reply is being generated.

n=$((n+1))
echo_i "checking that priming queries are not forwarded ($n)"
ret=0
nextpart ns7/named.run >/dev/null
dig_with_opts +noadd +noauth txt.example1. txt @10.53.0.7 > dig.out.$n.f7 || ret=1
start_pattern="sending packet to 10\.53\.0\.1"
retry_quiet 5 wait_for_log ns7/named.run "$start_pattern" || ret=1
check_sent 1 ns7/named.run "$start_pattern" ";\.[[:space:]]*IN[[:space:]]*NS$" || ret=1
sent=$(grep -c "10.53.0.7#.* (.): query '\./NS/IN' approved" ns4/named.run)
[ "$sent" -eq 0 ] || ret=1
sent=$(grep -c "10.53.0.7#.* (.): query '\./NS/IN' approved" ns1/named.run)
[ "$sent" -eq 1 ] || ret=1
if [ $ret != 0 ]; then echo_i "failed"; fi
status=$((status+ret))

ns7/named.run

12-Mar-2020 23:37:37.319 query client=0xe7d00758 thread=0xf49f3b40 (txt.example1/TXT): qctx_init
12-Mar-2020 23:37:37.327 sending packet to 10.53.0.1#8200
12-Mar-2020 23:37:37.347 received packet from 10.53.0.1#8200

[beetle:tests/system/forward] marka% grep -c "10.53.0.7#.* (.): query '\./NS/IN' approved" ns4/named.run
0
[beetle:tests/system/forward] marka% 

[beetle:tests/system/forward] marka% grep "10.53.0.7#.* (.): query '\./NS/IN' approved" ns1/named.run
12-Mar-2020 23:37:37.347 client @0xe7900758 10.53.0.7#39634 (.): query './NS/IN' approved
[beetle:tests/system/forward] marka%